THE AVONIAN OF THE AVON GORGE 
85 
that the base of the ‘ Carboniferous Limestone ’ in the North 
Wales area is of Upper Seminula age and belongs, therefore, to a 
very much later date in the earth’s history than the base of 
the Carboniferous Limestone in the Avon sequence. 
The statement that a particular species occurs at a cer- 
tain level in the ' Carboniferous Limestone ’ of one locality 
affords no clue to its position in the Carboniferous Lime- 
stone ’ of a second locality. On the other hand, the statement 
that a certain fossil occurs in a particular zone of the Avonian 
fixes its relative position for all localities. 
^ j It will not, I hope, be wasted space to close this short 
of an outline of zonal method with a tabular summary of 
Avonian gome of the more important incidents which marked 
Time-Scale <• a • . 
the progress of Avonian time. 
The particular incidents which I have elected to present are 
the times of first establishment of certain important genera and 
gentes of Corals and Brachiopods ; but, to avoid overcrowding, 
I have selected only one Brachiopod-genus, namely, Productus. 
